Dr. Walter Willett is the chair of nutrition at the Harvard School of Public Health. He's also a cow's best friend.

Earlier this month, Willett and colleagues, who have studied the link between diet and health for decades, published a study that followed more than 100,000 people over more than 20 years — and found that the amount of red meat they ate was linked to a rise in risk of premature death.

The notion that red meat might not be so great for you isn't exactly new, but carnivores cried foul. Willett answered a few of the critics this week.

Can you summarize briefly what you saw here? And what's the takeaway — is it OK to eat meat or not?

We looked at total mortality. And it wasn't too surprising that we did see a linear, step-wise increase in risk of dying prematurely with higher red meat consumption.

We went another step, and we compared one serving of red meat to other major protein sources, like poultry and fish and legumes. In every instance there was an advantage to consuming something else instead of red meat.

Individuals make their choices on the basis of many factors, health outcomes being one: religious beliefs, concerns about the environment, many things. But focusing in on health, it does appear that the data are quite strong.

There's no sharp cutoff. But when you get down to maybe one serving of meat or less per week the risk gets pretty low. If you really want to go for the lowest possible, it does look like not consuming red meat at all, or a couple times a year, is where you'd want to be.

This made a lot of people pretty angry and upset. One thing they wanted to know was,are you funded by PETA or the chicken industry? Another was, are you a vegan or vegetarian?

No, we are not funded by any of those industry sources. These studies are funded by the National Institutes of Health.

I eat poultry and I eat fish. Maybe a few times a year, I eat red meat. That certainly wasn't always my habit. I grew up in the Midwest, and I think probably I had red meat three times a day, like some of the people in the study. But I've seen the data come in.

This study relies on surveys. Maybe people don't remember what they ate.

In principle, the ideal study would take 100,000 people and randomly assign some to eating several servings of red meat a day and randomize the others to not consume red meat and then follow them for several decades. But that study, even with any amount of money, in many instances is simply not possible to do. Most people don't want to stay on any prescribed diet, particularly if they're living in an environment where other people around them are eating other things.

How about grass-fed beef? Does the way the animals are raised make a difference?

We don't know for sure. In this study we were investigating red meat as it is consumed in the United States, which is mostly lot-fed, grain-fed beef. I'm quite sure we would not have enough people consuming mostly grass-fed beef to be able to look at that on its own.

There are some differences. Omega-3 fatty acid levels are somewhat higher in the grass-fed beef. But if someone's getting other sources of omega-3 fatty acids — if they're having fish once or twice a week — the additional amount probably won't make too much difference.

The total fat may be a bit lower. But we don't see that the fat per se is really related to the risk of getting heart disease or cancer. Cholesterol is more in the lean part of the red meat, so that's going to be just as high and maybe even higher in the grass-fed animals.

I think it would be nice to be able to study grass-fed beef directly, but I think in the meantime it's reasonable to assume that the answer is probably not going to be very different from what we saw here.

We also got emails that said, "My Aunt Bertha ate meat three times a day her whole life and lived to be 95!"

Of course, yes. You can find people who smoked a pack of cigarettes a day and lived to be 100, too. Any one example doesn't make the case.

It is pretty clear that people back in the 1950s, eating all this red meat, were not living very long compared to how long we're living today. Red meat consumption has gone down; poultry has gone up. There has been a general shift in a better direction in our diets.

Red meat is not the whole picture, but the reduction probably has been a contributor to the reduction in mortality rates that we have today.

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